You make an untenable hidden assumption that the outside creator has to be a being off some kind.

But no being is necessary. It is still a possibility, but not a necessity, and your logic demands that it be a necessity.

Mathematics is a counter example, another alternative, if you will, that seems much more plausible to most of the posters on this forum, but is still just another possibility.

So there is no logical proof for either an outside being or mathematics being the creator of the universe.. My guess is that it is both and perhaps more that we have not imagined yet. If there is a god, then that god is not limited by our imagination, but your logic is.